To find the area of a rectangular countertop, you can use the formula:
\[ \text{Area} = \text{length} \times \text{width} \]
For a countertop that is 5 feet long and 3 feet wide, the calculation would be:
\[ \text{Area} = 5 , \text{feet} \times 3 , \text{feet} = 15 , \text{square feet} \]
Thus, the area of the countertop is \(15\) square feet.